How to access properties of the 3d Viewport?
Operators and properties are often context sensitive. It means they need to be executed from specific context (screen area). Your example needs to be run from View_3D context, so it cannot run from the Python Console.
Luckily, there is a way to access different screen areas and get their types and properties. In your case use_occlude_geometry is a property of SpaceView3D, thus only accessible from that space:
for area in bpy.context.screen.areas:
    if area.type == 'VIEW_3D':
        if bpy.context.active_object.mode == 'EDIT':
            area.spaces[0].use_occlude_geometry = not area.spaces[0].use_occlude_geometry

You can paste and run the above example in Python Console as a multi-line command or run it from the Text Editor as usual.

However, best practice for Addons is writing your own Operator to match the context:
import bpy

class OccludeGeometryToggle(bpy.types.Operator):
    &quot;&quot;&quot;Toggle Occlude Geometry Option in 3D View&quot;&quot;&quot;
    bl_idname = &quot;view3d.toggle_occlude_geometry&quot;
    bl_label = &quot;Occlude Geometry Toggle&quot;
    bl_options = {'REGISTER', 'UNDO'}

    @classmethod
    def poll(cls, context):
        return context.active_object.mode == 'EDIT'
    
    def execute(self, context):
        context.space_data.use_occlude_geometry = \
            not context.space_data.use_occlude_geometry
        return {'FINISHED'}

def register():
    bpy.utils.register_class(OccludeGeometryToggle)

def unregister():
    bpy.utils.unregister_class(OccludeGeometryToggle)

if __name__ == &quot;__main__&quot;:
    register()

Once the operator is registered, press Space in Edit Mode (since the poll() method checks whether the active object is in the correct mode) and type &quot;Occlude Geometry Tog...&quot;.

Note that the code above used to work till Blender 2.79. Since version 2.8+, the property use_occlude_geometry was removed from the API. A similar effect can be achieved by the property shading.show_xray -- it's not exactly the same thing though, because it also makes grid lines peek trough the object.